Embodiments relate generally to the automated testing, optimization and harmonization of the performance measurements of visual displays. More specifically, exemplary embodiments provide a system and method for efficiently determining the ideal Vcom for a liquid crystal display.
Embodiments may test, optimize and harmonize an active matrix liquid crystal display (AMLCD). AMLCD's are well known in the art, and depend on thin film transistors (TFT's) and capacitors to maintain an isolated charge at each subpixel until the next refresh cycle. They are arranged in a matrix on one of the glass panels between which is sandwiched the liquid crystal material. To address a particular subpixel, a gate voltage is applied to a row, switching on that row's transistors and thereby letting that row's subpixels accept a charge. Voltages (“gray level voltages”) are applied to the columns corresponding to the light transmission level desired at individual subpixel elements at the intersection of the column and row in question. Since the other rows that the column intersects are turned off, only the capacitor at the designated subpixel receives a charge from a particular column.
The voltage potential differential between the front glass panel and a subpixel TFT controls the amount of “untwisting” accomplished by the twisted nematic liquid crystalline material at the subpixel element. This level of untwisting, in turn, determines the amount of light, which the material permits to pass through the front glass panel. By controlling the voltage applied to the subpixels, LCD's can create a gray scale. In one type of LCD monitor the liquid crystals organize into a structure that makes the subpixels transparent in the absence of a voltage differential.
A net voltage potential should not be maintained across the cell gap between the glass plates for an appreciable time or electroplating of the liquid crystalline material will occur, and image retention will result. A variety of driving schemes are known in the field to avoid the said electroplating phenomenon. One way to avoid electroplating is to minimize the voltage potential being maintained across the cell gap by supplying an alternating polarity voltage potential to each subpixel TFT relative to the common voltage of the opposite plate (Vcom).
With respect to the alternating voltage potentials applied to the subpixel TFT's, if the magnitude of the positive and negative potentials at the subpixels relative to Vcom are different the light transmission level will appear to flicker as the panel refreshes. This flickering occurs because the liquid crystal switches from one orientation to the opposite depending on the polarity of the potential, and the magnitude of light transmission is determined by the magnitude of that potential. If the magnitude of the positive potential differs from the magnitude of the negative potential, the light transmission changes as the waveform changes from positive to negative, and vice versa. This “unbalanced” state resulting in flicker increases the likelihood of electroplating since a nonzero voltage potential is effectively maintained across the cell gap. “Harmonizing” an LCD display implies balancing, or correcting, this unbalanced state.
By electrically balancing, or harmonizing, a panel to a high degree of accuracy, the present invention prevents image retention, as described above, and allows for the setting of the optimum, or maximum, voltage potential range, resulting in, among other characteristics, maximum contrast ratio and maximum luminance, or light transmission level. Additionally, flicker is minimized. Through automation, the present invention provides for a time-efficient and highly repeatable method of selecting the ideal Vcom for the display under test (DUT).
Currently, systems are available to automatically test visual displays by providing measurements on display characteristics (for example: luminance, transmission level, contrast ratio, luminance uniformity, chromaticity uniformity, viewing angle dependence, and luminous efficiency) of the visual displays. For example, U.S. Pat. No. 6,809,746 provides a system for the optimization of display characteristics and that patent is incorporated by reference in its entirety herein. While the teachings of the '746 patent may be used to determine Vcom, this method is time-consuming and requires the use of an expensive testing chamber as well as expensive equipment. Exemplary embodiments do not require a testing chamber, can be performed quickly and easily, and involve relatively inexpensive and widely available equipment.
Furthermore, existing testing methods measure the flicker at only small areas on the display. Thus, many measurements across the front of the display must be made in order to determine the best Vcom for the display. This process becomes extremely time consuming, especially for large displays. Exemplary embodiments utilize a diffuser unit so that the flicker within a region on the display surface may be analyzed, rather than a small group of pixels or subpixels. Further embodiments allow the overall flicker of the entire display to be measured at once.
The exemplary embodiments herein disclosed are not intended to be exhaustive or to unnecessarily limit the scope of the invention. The exemplary embodiments were chosen and described in order to explain the principles of the invention so that others skilled in the art may practice the invention. Having shown and described exemplary embodiments, those skilled in the art will realize that many variations and modifications may be made to affect the described invention. Many of those variations and modifications will provide the same result and fall within the spirit of the exemplary embodiments. It is the intention, therefore, to limit the embodiments only as indicated by the scope of the claims.